The cellular communication system enables the reuse of spectrum resources, which brings the flourishing development of the wireless communication technology. In the cellular system, when the service needs to be transmitted between two user equipment (UE), the service data from a user equipment 1 (UE1) to a user equipment 2 (UE2) are firstly transmitted to a base station 1 through an air interface, the base station 1 transmits the service data to a base station 2 via a core network, and the base station 2 transmits the above-mentioned service data to the UE2 through the air interface. The transmission of the service data from the UE2 to the UE1 uses the similar processing flow. FIG. 1 is a schematic diagram of cellular communications when the UEs are located in the cell(s) of the same base station according to the related art. As shown in FIG. 1, when the UE1 and the UE2 are located in the cell of the same base station, although the base station 1 and the base station 2 are actually the same station, one data transmission would still consume two sets of wireless spectrum resources.
It can be seen that if the UE1 and the UE2 are located in the same cell and are proximate, then the above-mentioned cellular communication scheme is obviously not the optimal one. However, in fact, with the development tendency of the diversification of mobile communication services, for example, social networks, electronic payment, etc. are applied in the wireless communication system more and more widely, the requirement for the service transmission among users which are close to each other is growing. Therefore, more attention is paid to the device-to-device (D2D) communication. FIG. 2 is a schematic diagram of a D2D communication system according to the related technologies. As shown in FIG. 2, D2D refers to that the service data are directly transmitted to a destination UE through the air interface by a source UE without being forwarded by a base station. This kind of communication mode is different from the traditional cellular communication mode. For users which are close to each other, D2D not only saves wireless spectrum resources but also reduces the data transmission pressure of the core network.
It can be seen from the above-mentioned analysis that there is an obvious difference between the D2D communication and the traditional cellular communication mode. In another aspect, for the UE, while performing D2D communication, the cellular transmission with a network side also possibly exists, and the design of the system in this case may be more complex. Under this assumption, how to avoid the conflict between the D2D communication and the cellular transmission of the UE also becomes a problem to be considered in the device-to-device communication research.